Hair dyes are classified depending upon the types of dyestuff used or whether or not the hair dyes have a melanin bleaching action. Typical examples of hair dyes include two-agent type permanent hair dyes consisting of a first agent containing an alkali agent, an oxidation dye and an optional direct dye such as a nitro dye and a second agent containing an oxidizing agent; and one-agent type semipermanent hair dyes containing an organic acid or an alkali agent and a direct dye such as an acid dye, a basic dye and a nitro dye.
Patent Document 1 proposes an azo dye containing a dissociative proton as a direct dye suitable for use in dyeing the hair and teaches that hair-dyeability is produced in an anionic state where a dissociative proton(s) dissociates in an alkaline condition.
(Patent Document 1) JP-A-2003-342139